In chronic immune and inflammatory diseases, a characteristic feature is the continued recruitment and accumulation in tissues of mononuclear leukocytes (lymphocytes and monocytes). Much evidence points to dysfunction and/or activation of endothelium as a critical factor underlying this process. Inflammatory cytokines and bacterial products induce localized endothelial cell activation and initiate endothelial-dependent mechanisms that mediate leukocyte firm adhesion and transmigration. Although much attention has been paid to leukocyte-endothelial adhesive interactions, only recently have the molecular mechanisms for leukocyte passage through the endothelial lateral junctions become a focus of study. Our laboratory and others have shown that leukocyte adhesion alters adherens junctions (e.g., VE-cadherin complex) at endothelial lateral borders, during the transmigration event (1-3). Junction Adhesion Molecule (JAM) is a recently identified molecule concentrated at cell-cell borders of murine endothelium and epithelium, and participates in monocyte transmigration across endothelial cells in vivo and in vitro (4). Preliminary studies presented in this application have identified the putative human JAM molecule. The overall goals of this application are to characterize the newly discovered human JAM adhesion pathway in the context of mononuclear leukocyte recruitment and function in endothelial cell lateral junctions. Specific Aim 1 will determine the expression of human JAM in resting and cytokine activated endothelium and the exact role in the leukocyte-endothelial adhesion cascade using live time videomicroscopy in an in vitro flow model. The experiments proposed in Specific 2 will determine the role of JAM in endothelial cell-to-cell adhesion (independent of leukocyte adhesion/transmigration) and association with other proteins localized to cell junctions or the cytoskeleton. Specific Aim 3 will utilize in vivo models of chronic inflammation or immune reactions to test the potential role of human JAM in leukocyte recruitment.